When the brakes of a vehicle are applied, a braking force between the wheel and the road surface is generated that is dependent upon various parameters including the road surface conditions and the amount of slip between the wheel and the road surface. The braking force increases as slip increases, until a critical value of slip is surpassed. Beyond this critical slip value, the braking force decreases and the wheel rapidly approaches lockup. If the wheel is allowed to lock, unstable braking occurs, and vehicle stopping distance on uniform nondeformable surfaces increases. Thus, stable vehicle braking occurs when wheel slip does not exceed this critical slip value. An antilock control system achieves stable braking and minimizes stopping distance by detecting incipient wheel lock. Criteria used to sense incipient wheel lock are excessive wheel deceleration and/or excessive wheel slip. Once an incipient wheel lock has been detected, pressure is relieved at the wheel brake. Upon releasing the brake pressure, the wheel will begin to recover from the incipient wheel lock condition. When the wheel has substantially recovered, brake pressure is reapplied. One criterion that is typically used to indicate wheel recovery is a positive wheel acceleration. Reapplication of brake pressure results in the wheel again approaching lockup and the wheel cycle process is repeated. Brake force and vehicle braking efficiency are maximized during braking by cycling the brake pressure around an optimum pressure for the particular road surface. The optimum pressure corresponds to the brake force generated while at the critical wheel slip value. Since the brake force is a function of wheel brake pressure and road surface conditions, the optimum brake force and the corresponding optimum brake pressure will change as road surface conditions vary. To optimize vehicle braking during a stop on a changing or non-uniform road surface, the antilock control system must be able to respond to each road surface and seek a new optimal pressure quickly to insure maximum braking efficiency. The U.S. Pat. No. 4,881,784 issued to Leppek discloses an example of such a system and is incorporated herein by reference.
When vehicle braking occurs on a road surface which has one coefficient of friction on one side of the vehicle and a markedly different coefficient on the other side, the surface is said to have a split coefficient. An example of this is a road which is covered with ice or snow along one side and is clear or dry near the center so that the right side wheels engage a low coefficient of friction and the left side wheels engage a high coefficient. The result of braking on such a surface is that the vehicle tends to yaw toward the high coefficient side. Most wheel lock control systems are designed to deal with the split coefficient surface in either of two ways if left and right wheels can be separately controlled. One approach is to independently control the brakes according to the optimum operation on each side. The result is that yaw occurs but stopping distance is minimized. The other approach is to control the brakes according to the optimum operation on the low coefficient side for a programmed period of time and then gradually resume independent brake control. This forces both brakes to have the same low pressure initially and increases vehicle stability but also increases the stopping distance.